The harmonic drive generally comprises a cylindrically-shaped internal gear, a cup-shaped flexible gear and a wave generator. The wave generator is generally in the form of an ellipse and is disposed on an inner side of said flexible gear. The flexile gear provided with said wave generator is disposed on an inner circumferential surface of said internal gear. An inner circumferential surface of the internal gear and an outer circumferential surface of the flexible gear are generally configured so that the surfaces each has a tooth profile machined to have no slippage between them.
In the harmonic drive, when the internal gear is fixed and the wave generator is rotated, due to a difference of circumference lengths of the flexible gear and the internal gear, a slight rotation occurs in the flexible gear, which is called a harmonic motion. The one that utilizes such a harmonic motion is the harmonic drive.
In a cup-shaped flexible gear, the part corresponding to the inlet of the cup must be flexible in order to make the harmonic motion possible, and the part corresponding to the bottom of the cup must be hard in order to create an output of the flexible gear. That is to say, a single component needs to have different physical properties, which causes designing and manufacture thereof difficult. For example, there is a limitation in reducing the length of the cup, and, as a result, the size of the harmonic drive becomes large.
In order to solve such a problem, a harmonic drive with a band type of flexible gear was developed. A further internal gear further engaging with the flexible gear is needed to make the rotation of the flexible gear an effective output, in which case the further internal gear is designed to have the same number of teeth as those of the band type of flexible gear so that a relative rotating motion between the further internal gear and the band type of flexible gear does not occur.
However, the band type of flexible gear is deformed by the wave generator which is in the form of an ellipse, whereby a relative radial motion between the further internal gear and the band type of flexible gear occurs. Namely, the relative radial motion occurs by the difference between a major axis and a minor axis of the ellipse-shaped wave generator. Such relative radial motion results in a backlash problem.
Among harmonic drives having a band type of flexible gear applied therein there is a harmonic drive utilizing a double harmonic motion, which is however in need of a set of gear teeth having a plurality of different pitch diameters in the flexible gear, thereby causing lots of manufacturing problems.